You must first fill in your company address. This ensures the system knows where your business is located.
If you entered a country in the EU, you will also be able to enter your EU VAT number. This is used to determine if tax will be charged to you or not. The value you enter will be validated, showing either a green tick or an "invalid" message. An invalid or missing EU VAT number will not prevent you being able to purchase credits, but the order will be processed as if you did not have a valid EU VAT number.